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Neoproterozoic to middle palaeozoic evolution of the Central Asian Orogenic Belt in south-central Mongolia [Elektronische Ressource] : chronological and geochemical perspectives / Antoine Demoux

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Neoproterozoic to middle Palaeozoic evolution of the Central Asian Orogenic Belt in south-central Mongolia: chronological and geochemical perspectives Dissertation zur Erlangung des Grades “Doktor der Naturwissenschaften” im Promotionsfach Geologie/Paläontologie am Fachbereich Chemie, Pharmazie und Geowissenschaften der Johannes Gutenberg-Universität Mainz Antoine Demoux geb. in Mâcon Mainz, 2009 Erklärung Ich versichere hiermit, die vorliegende Arbeit selbständig und nur unter Verwendung der angegebenen Quellen und Hilfsmittel verfasst zu haben. All views and results presented in this thesis are those of the author, unless otherwise stated. Mainz, May 2009 Abstract Abstract Mongolia occupies a central position within the eastern branch of the large accretionary Central Asian Orogenic Belt (CAOB) or Altaids. The present work aims to outline the geodynamic environment and possible evolution of this part of the eastern CAOB, predominantly from the Cambrian to the middle Palaeozoic. The investigation primarily focussed on zircon geochronology as well as whole-rock geochemical and Sm–Nd isotopic analyses for a variety of metaigneous rocks from the southern Hangay and Gobi-Altai regions in south-central Mongolia. The southern slope of the Hangay Mountains in central Mongolia exposes a large NWSE-trending middle Neoproterozoic ophiolitic complex (c.

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Published 01 January 2009
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Neoproterozoic to middle Palaeozoic evolution of the Central
Asian Orogenic Belt in south-central Mongolia:
chronological and geochemical perspectives

Dissertation
zur Erlangung des Grades
“Doktor der Naturwissenschaften”
im Promotionsfach Geologie/Paläontologie

am Fachbereich Chemie, Pharmazie und Geowissenschaften
der Johannes Gutenberg-Universität Mainz

Antoine Demoux
geb. in Mâcon

Mainz, 2009
Erklärung

Ich versichere hiermit, die vorliegende Arbeit selbständig und nur unter Verwendung der
angegebenen Quellen und Hilfsmittel verfasst zu haben.

All views and results presented in this thesis are those of the author, unless otherwise
stated.

Mainz, May 2009
Abstract
Abstract
Mongolia occupies a central position within the eastern branch of the large accretionary
Central Asian Orogenic Belt (CAOB) or Altaids. The present work aims to outline the
geodynamic environment and possible evolution of this part of the eastern CAOB,
predominantly from the Cambrian to the middle Palaeozoic. The investigation primarily
focussed on zircon geochronology as well as whole-rock geochemical and Sm–Nd isotopic
analyses for a variety of metaigneous rocks from the southern Hangay and Gobi-Altai regions
in south-central Mongolia.
The southern slope of the Hangay Mountains in central Mongolia exposes a large
NWSE-trending middle Neoproterozoic ophiolitic complex (c. 650 Ma), which is
tectonically integrated within an accretionary complex developed between the Precambrian
Baydrag and Hangay crustal blocks. Formation of the entire accretionary system along the
north-eastern margin of the Baydrag block mainly occurred during the early Cambrian, but
convergence within this orogenic zone continued until the early Ordovician, because of on-
going southward subduction-accretion of the Baydrag block.
An important discovery is the identification of a late Mesoproterozoic to early
Neoproterozoic belt within the northern Gobi-Altai that was reworked during the late
Cambrian and throughout the late Ordovician/Devonian. Early Silurian low-grade mafic and
felsic metavolcanic rocks from the northern Gobi-Altai display subduction-related
geochemical features and highly heterogeneous Nd isotopic compositions, which suggest an
origin at a mature active continental margin. Early Devonian protoliths of granodioritic and
mafic gneisses from the southern Gobi-Altai display geochemical and Nd isotopic
compositions compatible with derivation and evolution from predominantly juvenile crustal
and mantel sources and these rocks may have been emplaced within the outboard portion of
the late Ordovician/early Silurian active continental margin. Moreover, middle Devonian low-
grade metavolcanic rocks from the southwestern Gobi-Altai yielded geochemical and Nd
isotopic data consistent with emplacement in a transitional arc-backarc setting.
The combined U–Pb zircon ages and geochemical data obtained from the Gobi-Altai
region suggest that magmatism across an active continental margin migrated oceanwards
through time by way of subduction zone retreat throughout the Devonian. Progressive
extension of the continental margin was associated with the opening of a backarc basin and
culminated in the late Devonian with the formation of a Japan-type arc front facing a
southward open oceanic realm (present-day coordinates).
iZusammenfassung
Zusammenfassung
Die Mongolei nimmt eine zentrale Position innerhalb des östlichen Teils des großen
Zentralasiatischen Orogens (CAOB), auch Altaiden genannt, ein. Die vorliegende Arbeit
beschreibt das geodynamische Umfeld und die Entwicklung dieses Teils des CAOB vom
Kambrium bis ins mittlere Paläozoikum. Die Untersuchungen konzentrieren sich auf
Zirkondatierungen sowie die Geochemie und Sm–Nd-Isotopie von Gesamtgesteinsproben
einer breiten Palette von metamorphen Orthogesteinen der südlichen Hangay und Gobi-Altai
Region im südlichen Teil der Mongolei.
An der südlichen Flanke des Hangay-Gebirges ist ein umfangreicher NW–SE streichender
Ophiolitkomplex aus dem mittleren Neoproterozoikum aufgeschlossen (c. 650 Ma). Dieser
wurde tektonisch in einen Akkretionskeil zwischen den präkambrischen Baydrag und Hangay
Krustenblöcken integriert. Die Hauptphase der Akkretion im gesamten System liegt im frühen
Kambrium, jedoch hält die konvergente Tektonik aufgrund der andauernden südwärts
gerichteten Akkretion des Baydrag-Grundgebirges bis ins frühe Oberordovizium an.
Von besonderer Bedeutung ist die Entdeckung eines Krustensegmentes innerhalb des
nördlichen Altai aus dem späten Mesoproterozoikum bis frühen Neoproterozoikum der im
oberen Kambrium und Ordovizium/Devon tektonisch und metamorph überprägt wurde.
Untersilurische niedriggradige mafische und felsische Metavulkanite aus dem nördlichen
Gobi-Altai zeigen subduktionstypische geochemische Eigenschaften und stark heterogene
Nd-Isotopie, die auf eine Entstehung an einem reifen aktiven Kontinentalrand hinweisen.
Frühdevonische Protolithe aus granodioritischen und mafischen Gneisen des südlichen Gobi-
Altai legen auf Grund ihrer Geochemie und Nd-Isotopie eine Herkunft und Entwicklung aus
hauptsächlich juveniler Kruste und Mantel nahe. Die Platznahme dieser Gesteine könnte
innerhalb der Randbereiche des spätordovizisch/frühsilurischen aktiven Kontinentalrandes
stattgefunden haben. Darüber hinaus deuten Geochemie und Nd-Isotopie mitteldevonischer
niedriggradiger Metavulkanite aus dem südwestlichen Gobi-Altai auf eine Platznahme in
einem “transitional back-arc” hin.
U–Pb Zirkonalter kombiniert mit geochemischen Daten aus der Gobi-Altai-Region legen
nahe, dass durch Zurückschreiten der Subduktionszone während des Devons der
Magmatismus über den aktiven Kontinentalrand in Richtung Ozean migrierte. Progressive
Extension des Kontinentalrandes war mit der Öffnung eines Back-Arc-Beckens verbunden
und gipfelte im späten Devon in der Ausbildung einer “Japan-type arc front” in Richtung
eines nach Südwesten offenen Ozeans (nach heutigen Koordinaten).
iiTable of Contents
Table of Contents

Abstract ......................................................................................................................................i
Zusammenfassung ....................................................................................................................ii
Preface ....................................................................................................................................... 1

Chapter 1 - Precambrian crystalline basement in southern Mongolia as revealed by
SHRIMP U–Pb zircon dating ................................................................................. 5
Abstract................................................................................................................................. 5
1. Introduction ...................................................................................................................... 6
2. Geological framework ...................................................................................................... 8
3. Sample description and geochronological results .......................................................... 10
4. Whole-rock major and trace elements chemistry ........................................................... 16
4.1. Major element chemistry.......................................................................................17
4.2. Trace element chemistry........................................................................................18
5. Discussion....................................................................................................................... 20
5.1. Precambrian thermal events and possible link to identified crustal
basement20
5.2. Late Cambrian plutonism and geodynamic implications ......................................22
6. Conclusions .................................................................................................................... 24

Chapter 2 - Zircon ages from the Baydrag block and the Bayankhongor ophiolite
zone: time constraints on late Neoproterozoic to Cambrian subduction-
and accretion-related magmatism in central Mongolia ........................................ 25
Abstract............................................................................................................................... 25
1. Introduction .................................................................................................................... 26
2. Regional geology and geochronological background of the Baydrag
block and Bayankhongor ophiolite zone ............................................................................ 28
2.1. Baydrag block........................................................................................................30
2.2. Burd Gol mélange..................................................................................................31
2.3. Southern volcanics.................................................................................................32
2.4. Bayankhongor ophiolite zone (BOZ) ....................................................................32
2.5. Dzag zone ..............................................................................................................33
2.6. Hangay zone ..........................................................................................................33
3. Field relationships along the Taats River ....................................................................... 34
4. Geochronological results ................................................................................................ 37
4.1. Analytical methods37
4.2. Baydrag block, Burd Gol mélange and Dzag zone ...............................................39
4.3. Taats River and Bayanleg areas.............................................................................42
5. Discussion....................................................................................................................... 47
5.1. Tectonothermal evolution of the Baydrag block prior to 1.8 Ga...........................47
v Table of Contents
5.2. Evidence for Meso- and Neoproterozoic episodes of zircon growth
and heterogeneity of the Baydrag block ...................................................................... 48
5.3. Late Neoproterozoic-Cambrian magmatism and regional implications ............... 49
5.4. Post-accretion Ordovician volcanic activity ......................................................... 51
5.5. Regional correlation with Cambro-Ordovician metamorphic and
magmatic events in western Mongolia and southern Tuva.......................................... 52
6. Conclusions.....................................................................................................................54

Chapter 3 - Middle Palaeozoic evolution of an active continental margin in southern
Mongolia: zircon ages and geochemical characteristics of igneous rocks
of the Gobi-Altai region........................................................................................55
Abstract ...............................................................................................................................55
1. Introduction56
2. Geological setting ...........................................................................................................58
3. Sample localities .............................................................................................................62
3.1. Halbagant Range ................................................................................................... 62
3.2. Hulsnii Valley (Bayanleg Hatuu Range) .............................................................. 63
3.3. Shine Jinst area...................................................................................................... 65
4. Analytical methods .........................................................................................................66
5. Petrography .....................................................................................................................69
6. U–Pb zircon geochronology and age interpretations ......................................................70
6.1. Bayanleg Hatuu Range.......................................................................................... 70
6.2. Hulsnii Valley (Bayanleg Hatuu Range) .............................................................. 72
6.3. Shine Jinst area 73
7. Whole-rock chemical compositions................................................................................75
7.1. Major and trace element data 75
7.1.1. Sample alteration............................................................................................ 75
7.1.2. Halbagant Range 76
7.1.3. Hulsnii Valley (Bayanleg Hatuu Range) ....................................................... 78
7.1.4. Shine Jinst area............................................................................................... 79
7.2. Sm–Nd isotopic data ............................................................................................. 81
8. Interpretation of geochemical and Nd isotopic data83
8.1. Mafic to intermediate metaigneous rocks ............................................................. 83
8.2. Felsic metaigneous rocks ...................................................................................... 86
9. Discussion .......................................................................................................................88
10. Conclusions...................................................................................................................92

Chapter 4 - Devonian arc-related magmatism in the Tseel terrane of southwestern
Mongolia: chronological and geochemical evidence............................................93
Abstract ...............................................................................................................................93
1. Introduction.....................................................................................................................94
2. Geological framework.....................................................................................................95
2.1. Tseel terrane.......................................................................................................... 96
vi Table of Contents
2.2. Geology of the sampled transect in the Devreh Valley.........................................98
3. Petrography..................................................................................................................... 99
4. U–Pb zircon ages.......................................................................................................... 100
5. Whole-rock major and trace element data.................................................................... 103
5.1. Sample alteration .................................................................................................104
5.2. Mafic metavolcanic rocks....................................................................................104
5.3. Felsic metaigneous rocks.....................................................................................105
5.4. Undeformed intrusive rocks ................................................................................107
6. SmNd isotopic data .................................................................................................... 107
7. Interpretation of geochemical and isotopic data........................................................... 109
7.1. Mafic metavolcanic rocks109
7.2. Felsic metaigneous rocks110
8. Discussion..................................................................................................................... 111
8.1. Geodynamic setting and evolution of the Tseel terrane ......................................111
8.2. Regional correlations for southern Mongolia......................................................112
8.3. Large-scale correlations with the Chinese Altai..................................................114
9. Conclusions .................................................................................................................. 115

References.............................................................................................................................. 117

Appendix ............................................................................................................................... 131







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